Components operating in the gas path environment of gas turbine engines are subjected to significant temperature extremes resulting from the changing power requirements during engine operation. In addition, the hot gases of combustion provide a corrosive and oxidative environment for these components. Without implementing protective mechanisms for the superalloys comprising these components, the alloys would have a shortened life span.
In order to protect these components from the deleterious effects of both the significant temperature extremes and the corrosive and oxidative environments at elevated temperatures, it has become common practice to coat these components with materials that are more capable of withstanding the hot, corrosive and oxidative environments. To provide these protections, environmental coatings typically are applied over the base metals comprising the components. The environmental coatings typically are metallic systems. These base metals typically are superalloys based on nickel, cobalt and combinations thereof, and the environmental coatings are typically aluminides or MCrAlYs. To improve the thermal performance of the component, thermal barrier coatings (TBCs) may be applied over the environmental coatings to form a thermal barrier system. Typically, these thermal barrier coatings are ytrria-stabilized zirconia (YSZ).
The environmental coatings may be applied by any one of a number of methods. One method is to expose the component at an elevated temperature to an atmosphere rich in preselected elements, typically aluminum. The environmental coating is then grown by a diffusion process as nickel or cobalt from the superalloy substrate diffuses outwardly from the substrate and Al diffuses inwardly to form an aluminum enriched region at the surface of the component. Another method is to apply a thin coating of an element such as platinum to the surface of the component prior to the aluminiding treatment by submersing the component in an electroplating bath of metallic ions of the element. Application of a voltage and the resulting current causes metallic ions to be deposited onto the component. The protective coating can be formed by a subsequent diffusion treatment. The third commonly used method for applying coatings is by thermal spray techniques. Thermal spray methods include detonation gun (D-gun), high velocity oxy-fuel (HVOF), air plasma spray (APS) and low pressure plasma spray (LPPS). The bond coat may also be applied by physical vapor depositing methods such as electron beam and magnetron sputtering. While the specific thermal spray methods vary, each of the methods does not significantly alter the surface of the substrate material by melting substrate material and mixing it with bond coat material. These thermal methods utilize a powder of a preselected composition, which is melted or partially melted and deposited on the substrate surface.
Substantially stoichiometric NiAl with small additions of other elements have been shown to exhibit outstanding oxidation resistance and resistance to TBC spallation. Such compositions are primarily beta phase NiAl and comprise substantially different compositions than have been applied conventionally in commercial applications. For example, pending patent application 13DV-13334, U.S. Ser. No. 09/608,114, filed Jun. 30, 2000, now U.S. Pat. No. 6,291,084, assigned to the assignee of the present invention discloses a xcex2-phase-NiAl having 30-60 atomic percent Al that includes small additions of Zr from, in atomic percent, from 0.1-1.2% and Cr from, in atomic percent 2-15%. This substantially stoichiometric NiAl has improved spallation resistance when used as a bond coat in a high temperature environment in which the component is subjected to severe thermal stresses and cycling, such as rotating components found in the hot sections of a gas turbine engine. Cr is included as a solid solution strengthener, but may also form fine xcex1-Cr phases and xcex2 Heusler phases disposed within the protective NiAl xcex2-phase. The small amounts of Cr are believed to promote the formation of xcex1-alumina at the surface of the bond coat. The small amount of Zr preferentially oxidizes at the coating surface forming xe2x80x9cpegsxe2x80x9d that increase the mechanical integrity of the alumina scale along the bond coat surface by creating an irregular or roughened surface, thereby increasing resistance to crack propagation. The application suggests that such coatings can possibly be applied by thermal spray, but teaches that the coatings are advantageously applied by electron beam physical vapor deposition (EBPVD) or by magnetron-sputtered PVD followed by a heat treatment in the range of 1800-2000xc2x0 F. for 24 hours to diffuse the coating with the underlying substrate.
Diffusion processes yield graded coatings in which the composition of the NiAl will vary from the outer, coated surface of the turbine component into the substrate of the turbine component. In order to take advantage of the improved oxidation resistance and resistance to TBC spallation while using existing technology, such beta-phase coatings of NiAl may best be applied using thermal spray techniques rather than using the PVD techniques suggested by the copending application. However, considerable difficulty may be encountered in attempting to apply such coatings to a turbine component. These difficulties may be traced to the processing variables inherent in the thermal spray processes, which in turn can affect the integrity of such substantially beta-phase NiAl-based coatings.
While PVD techniques are available for application of a beta-phase NiAl, what is lacking in the art are teachings with regard to thermal spray methods for depositing a composition of primarily beta-phase NiAl, with optional additions of rare earth and other elements, that will provide a bond coat with a fully developed metallurgical bond with minimal diffusion between the bond coat and the substrate that has substantial cleanliness with very little porosity while providing outstanding resistance to oxidation and resistance to TBC spallation.
Environmental coatings comprising primarily beta-phase NiAl with small additions of rare earth elements provide outstanding oxidation resistance to turbine components to provide additional environmental protection. As used herein, the term xe2x80x9csubstantially stoichiometric NiAlxe2x80x9d means a primarily beta-phase NiAl composition that includes small additions of rare earth elements and other elements such as Cr, such that the beta phase matrix may include small amounts of other phases, such as fine xcex1-Cr and xcex2xe2x80x2-Heusler phases. Frequently these applied environmental coatings are also used as bond coat to promote improved adhesion between dissimilar metallic substrates and ceramic thermal barrier coatings that are applied to improve the thermal response of the component. Substantially stoichiometric NiAl also provides outstanding resistance to TBC spallation. These coatings are applied to hot section turbine components by providing a powder of the substantially stoichiometric material with the desired minor additions of rare earth elements, Cr or Zr. The coatings are applied by a thermal spray process utilizing hydrogen as a fuel. The thermal spray melts the powder and directs it onto the surface of the turbine component that is to be coated. Thus, the process is accomplished by adding material to the surface of the substrate component, typically a cobalt-based or nickel-based superalloy component. Because the thermal spray process utilizing hydrogen as a fuel minimally affects the alloy substrate by melting, the added material is bonded to the surface by a combination of metallurgical and mechanical bonding. It is necessary to provide a subsequent heat treatment to fully develop the metallurgical bond between the thermally sprayed coat and the underlying substrate.
The coating process must be carefully controlled to assure that a high quality coating is achieved. To achieve a high quality coating by thermal spray, it is important to control the processing parameters of the thermal spray. The flame conditions are controlled using a non-oxidizing hydrogen flame to prevent detrimental oxidation of the coating. The powder size is screened to provide a preselected size powder in the range of 10 to 44 microns. Coating thickness is controlled to provide an added preselected thickness between about 0.1 mil to about 12 mils. Because the thermal sprays can provide a somewhat rough surface finish, the coating performance is improved by reducing the surface roughness to provide a smoother surface profile. The subsequent heat treatment can also affect the ability of the coating to form a sound metallurgical bond with the substrate. Thus, the parameters of the subsequent heat treatment must be carefully controlled.
When each of the above parameters are carefully controlled, a high quality, substantially stoichiometric NiAl coating advantageously can be achieved. Such a coating provides the additional advantage of having an improved furnace cycle life, which is a well-accepted indication of improved length of life when in service in a gas turbine engine.
Another advantage of the present invention is that the processes used can be applied to both new engine components and to refurbish components removed from service in a gas turbine engine.
Another advantage of the present invention is that thermal spraying procedures of the present invention can be utilized to produce turbine airfoil components having the same spallation resistance more quickly and less expensive than other available processes.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.